The present invention relates to the field of non-cryogenic separation of gases into components, and provides a partially coated membrane which enhances the efficiency of the gas separation process.
It has been known to use a polymeric membrane to separate air into components. Various polymers have the property that they allow different gases to flow through, or permeate, the membrane, at different rates. A polymer used in air separation, for example, will pass oxygen and nitrogen at different rates. The gas that preferentially flows through the membrane wall is called the “permeate” gas, and the gas that tends not to flow through the membrane is called the “non-permeate” gas. The selectivity of the membrane is a measure of the degree to which the membrane allows one component, but not the other, to pass through.
A membrane-based gas separation system has the inherent advantage that the system does not require the transportation, storage, and handling of cryogenic liquids. Also, a membrane system requires relatively little energy. The membrane itself has no moving parts; the only moving part in the overall membrane system is usually the compressor which provides the gas to be fed to the membrane.
A gas separation membrane unit is typically provided in the form of a module containing a large number of small, hollow fibers made of the selected polymeric membrane material. The module is generally cylindrical, and terminates in a pair of tubesheets which anchor the hollow fibers. The tubesheets are impervious to gas. The fibers are mounted so as to extend through the tubesheets, so that gas flowing through the interior of the fibers (known in the art as the bore side) can effectively bypass the tubesheets. But gas flowing in the region external to the fibers (known as the shell side) cannot pass through the tubesheets.
In operation, a gas is introduced into a membrane module, the gas being directed to flow through the bore side of the fibers. One component of the gas permeates through the fiber walls, and emerges on the shell side of the fibers, while the other, non-permeate, component tends to flow straight through the bores of the fibers. The non-permeate component comprises a product stream that emerges from the bore sides of the fibers at the outlet end of the module.
The effectiveness of a membrane in gas separation depends not only on the inherent selectivity of the membrane, but also on its capability of handling a sufficiently large product flow. Gas permeates through the membrane due to the pressure differential between one side of the membrane and the other. Thus, to maintain the pressure differential, it is advantageous to remove the permeate gas from the vicinity of the fibers, after such gas has emerged on the shell side. Removal of the permeate gas maximizes the partial pressure difference across the membrane, with respect to the permeate gas, along the length of the module, thus improving both the productivity and recovery of the module. In the membrane module of the present invention, the permeate gas is made to flow out of the module in a direction opposite to that of the basic feed stream.
It has been known that certain materials, when coated onto a polymeric membrane, can enhance the selectivity of the membrane with regard to specific types of gases to be separated. U.S. Pat. No. 5,141,530, the disclosure of which is incorporated by reference herein, describes a solution, in water, of a non-ionic surfactant which, when applied to the membrane, improves its selectivity for the separation of air into oxygen and nitrogen, and thereby increases the efficiency of the air separation process. However, coating the membrane reduces the permeability of the membrane, so that while the coated membrane becomes more selective, it also processes less gas than an uncoated membrane.
In the above-cited patent, the entire lengths of the membrane fibers in the module are coated, so as to provide more selectivity. But although the product recovery is improved, the product flow rate is reduced, thus impairing the utility of the system. If one used, instead, an uncoated module, the product flow would be improved, but the product recovery would be reduced. Also, an uncoated module will produce a higher pressure drop, along the length of the fiber, as compared with a coated module.
The present invention provides a gas separation membrane module which maximizes the product recovery of the module, but which minimizes the pressure drop along the length of the module, for a given product flow. The module of the present invention therefore enjoys advantages of both a coated and an uncoated module, and combines the best features of both. The invention also includes a method of making the improved module.